Polypropylene resins are extensively used in the fields of automotive parts and other various molded articles including parts for domestic electrical appliances, miscellaneous goods, and films, because molded articles obtained therefrom are excellent in rigidity and high-temperature rigidity. However, the molded articles of general polypropylene resins have insufficient impact resistance and are hence usable in a limited range of applications.
Several methods are known as techniques for improving the impact resistance of polypropylene resins. Representative examples thereof include a technique in which propylene is block-copolymerized with ethylene and a technique in which an ethylene-.alpha.-olefin copolymer rubber is incorporated into a polypropylene resin. The polypropylene resins obtained by these techniques are used mainly in applications such as materials for exterior automotive parts such as automotive bumpers and materials for interior automotive parts, such as instrument panels, console boxes, resin pillars, and trims, and as materials for domestic electrical appliances, such as washing tubs, the housings of vacuum cleaners, and the housings of coffee makers. According to need, these polypropylene resins are used after incorporation of an inorganic filler, e.g., talc, thereinto or after addition of a rubber to the block copolymer. Although the impact resistance of polypropylene resin compositions has been improved due to these techniques, the compositions thus improved have new problems, for example, that when a rubber is added in a large amount, the molded articles obtained are reduced in rigidity and high-temperature rigidity and the compositions have reduced flowability. There is hence a desire for the development of a polypropylene resin composition which gives molded articles having improved impact resistance while retaining intact rigidity, high-temperature rigidity, and flowability.
For eliminating the above problem, several proposals have been made. Known as such proposals are: a technique in which a propylene block copolymer improved in the stereoregularity of the propylene homopolymer block is used in order to enhance the reduced rigidity and high-temperature rigidity (Examined Japanese Patent Publication No. 1-254706 and Unexamined Published Japanese Patent Application No. 62-187707); and a technique in which an inorganic filler, e.g., talc, is incorporated into a polypropylene resin. The technique in which the stereoregularity of a propylene homopolymer block is improved is used in various fields because it can improve rigidity and high-temperature rigidity and hardly increases the specific gravity of the composition. On the other hand, the technique in which talc is incorporation is also used frequently. However, problems have been pointed out, for example, that the incorporation of talc not only results in molded articles having an increased specific gravity and hence in increased product weights, but also impairs the flowability of the resultant composition itself.
For the purpose of improving impact resistance, resistance to impact blushing, etc., several techniques have been proposed in which various copolymers are incorporated into a polypropylene resin. For example, Examined Japanese Patent Publication No. 59-37294 discloses a resin composition obtained by incorporating a hydrogenated polybutadiene into a polypropylene resin, and Examined Japanese Patent Publication No. 62-45883 discloses a diblock copolymer comprising a hydrogenated polybutadiene. Furthermore, Unexamined Published Japanese Patent Applications Nos. 4-342752 and 5-132606 disclose a resin composition obtained by incorporating a block copolymer comprising a hydrogenated polybutadiene into a polypropylene resin, and Unexamined Published Japanese Patent Applications Nos. 1-168743 and 1-168744 disclose a resin composition obtained by incorporating a hydrogenated block copolymer of isoprene and butadiene into a polypropylene resin or polyethylene resin.
The above resin compositions generally can have both improved resistance to impact blushing or blushing by bending and improved impact resistance. However, these resins have a problem, for example, that their rigidity and high-temperature rigidity are considerably low for the improvement in impact resistance. Practical applications thereof are hence limited.
When a rubber ingredient such as those described above, which serves to improve the impact resistance of a polypropylene resin composition, is added in a large amount, new problems arise. For example, the resultant composition gives molded articles having reduced rigidity and has impaired flowability. Consequently, attention is focused on the development of a polypropylene resin composition which can give molded articles having improved impact resistance without impairing the rigidity of the molded articles or reducing the flowability of the resin composition. An object of the present invention is to provide polyolefin resin compositions having excellent flowability and giving molded articles excellent in rigidity and impact resistance and in balance between these properties.
The present inventors made intensive investigations in order to eliminate the problems described above. As a result, they have found that a polyolefin resin composition having excellent flowability and giving molded articles having an excellent balance between rigidity and impact resistance is obtained by incorporating a hydrogenated diene block copolymer into a composition comprising an ethylene-(higher .alpha.-olefin) copolymer rubber and a high-rigidity polypropylene/propylene-ethylene block copolymer and optionally containing talc. The present invention has been completed based on this finding.